SM+5220+COD_20th+ed

5220 CHEMICAL OXYGEN DEMAND (COD)*

5220 A. Introduction

Chemical oxygen demand (COD) is defined as the amount of aspecified oxidant that reacts with the sample under controlled con-ditions. The quantity of oxidant consumed is expressed in terms ofits oxygen equivalence. Because of its unique chemical properties,the dichromate ion (Cr2O72) is the specified oxidant in Methods5220B, C, and D; it is reduced to the chromic ion (Cr3) in thesetests. Both organic and inorganic components of a sample aresubject to oxidation, but in most cases the organic componentpredominates and is of the greater interest. If it is desired to measureeither organic or inorganic COD alone, additional steps not de-scribed here must be taken to distinguish one from the other. CODis a defined test; the extent of sample oxidation can be affected bydigestion time, reagent strength, and sample COD concentration.

COD often is used as a measurement of pollutants in wastewaterand natural waters. Other related analytical values are biochemicaloxygen demand (BOD), total organic carbon (TOC), and totaloxygen demand (TOD). In many cases it is possible to correlate twoor more of these values for a given sample. BOD is a measure ofoxygen consumed by microorganisms under specific conditions;TOC is a measure of organic carbon in a sample; TOD is a measureof the amount of oxygen consumed by all elements in a samplewhen complete (total) oxidation is achieved.

In a COD analysis, hazardous wastes of mercury, hexavalentchromium, sulfuric acid, silver, and acids are generated. Meth-ods 5220C and D reduce these waste problems but may be lessaccurate and less representative. (See 2 below.)

1. Selection of Method

The open reflux method (B) is suitable for a wide range of wasteswhere a large sample size is preferred. The closed reflux methods (Cand D) are more economical in the use of metallic salt reagents andgenerate smaller quantities of hazardous waste, but require homog-enization of samples containing suspended solids to obtain repro-ducible results. Ampules and culture tubes with premeasured re-agents are available commercially. Measurements of sample vol-umes as well as reagent volumes and concentrations are critical.Consequently, obtain specifications as to limits of error for pre-mixed reagents from manufacturer before use.

Determine COD values of 50 mg O2/L by using procedures5220B.4a, C.4, or D.4. Use procedure 5220B.4b to determine,with lesser accuracy, COD values from 5 to 50 mg O2/L.

2. Interferences and Limitations

Oxidation of most organic compounds is 95 to 100% of thetheoretical value. Pyridine and related compounds resist oxidationand volatile organic compounds will react in proportion to theircontact with the oxidant. Straight-chain aliphatic compounds areoxidized more effectively in the presence of a silver sulfate catalyst.

The most common interferent is the chloride ion. Chloride reactswith silver ion to precipitate silver chloride, and thus inhibits thecatalytic activity of silver. Bromide, iodide, and any other reagentthat inactivates the silver ion can interfere similarly. Such interfer-ences are negative in that they tend to restrict the oxidizing action ofthe dichromate ion itself. However, under the rigorous digestionprocedures for COD analyses, chloride, bromide, or iodide can reactwith dichromate to produce the elemental form of the halogen andthe chromic ion. Results then are in error on the high side. Thedifficulties caused by the presence of the chloride can be overcomelargely, though not completely, by complexing with mercuric sul-fate (HgSO4) before the refluxing procedure. Although 1 g HgSO4is specified for 50 mL sample, a lesser amount may be used wheresample chloride concentration is known to be less than 2000 mg/L,as long as a 10:1 weight ratio of HgSO4:Cl is maintained. Do notuse the test for samples containing more than 2000 mg Cl/L.Techniques designed to measure COD in saline waters areavailable.1,2

Halide interferences may be removed by precipitation withsilver ion and filtration before digestion. This approach mayintroduce substantial errors due to the occlusion and carrydownof COD matter from heterogenous samples.

Ammonia and its derivatives, in the waste or generated fromnitrogen-containing organic matter, are not oxidized. However,elemental chlorine reacts with these compounds. Hence, correc-tions for chloride interferences are difficult.

Nitrite (NO2) exerts a COD of 1.1 mg O2/mg NO2-N.Because concentrations of NO2 in waters rarely exceed 1 or 2mg NO2-N/L, the interference is considered insignificant andusually is ignored. To eliminate a significant interference due toNO2, add 10 mg sulfamic acid for each mg NO2-N present inthe sample volume used; add the same amount of sulfamic acidto the reflux vessel containing the distilled water blank.

Reduced inorganic species such as ferrous iron, sulfide, man-ganous manganese, etc., are oxidized quantitatively under thetest conditions. For samples containing significant levels of thesespecies, stoichiometric oxidation can be assumed from knowninitial concentration of the interfering species and correctionscan be made to the COD value obtained.

The silver, hexavalent chromium, and mercury salts used inthe COD determinations create hazardous wastes. The greatestproblem is in the use of mercury. If the chloride contribution toCOD is negligible, HgSO4 can be omitted. Smaller sample sizes(see 5220C and D) reduce the waste. Recovery of the wastematerial may be feasible if allowed by regulatory authority.3

3. Sampling and Storage

Preferably collect samples in glass bottles. Test unstable sam-ples without delay. If delay before analysis is unavoidable,preserve sample by acidification to pH 2 using conc H2SO4.Blend (homogenize) all samples containing suspended solidsbefore analysis. If COD is to be related to BOD, TOC, etc.,ensure that all tests receive identical pretreatment. Make prelim-

* Approved by Standard Methods Committee, 1997.Joint Task Group: 20th EditionClarence G. Johnson (chair), Donald G. Miller,John T. Pivinski.

5-14 AGGREGATE ORGANIC CONSTITUENTS (5000)

inary dilutions for wastes containing a high COD to reduce theerror inherent in measuring small sample volumes.

4. References

1. BURNS, E.R. & C. MARSHALL. 1965. Correction for chloride interfer-ence in the chemical oxygen demand test. J. Water Pollut. ControlFed. 37:1716.

2. BAUMANN, F.I. 1974. Dichromate reflux chemical oxygen demand: Aproposed method for chloride correction in highly saline waters.Anal. Chem. 46:1336.

3. HOLM, T.R. 1996. Treatment of Spent Chemical Oxygen DemandSolutions for Safe Disposal. Illinois State Water Survey, Champaign.

5220 B. Open Reflux Method

1. General Discussion

a. Principle: Most types of organic matter are oxidized by aboiling mixture of chromic and sulfuric acids. A sample isrefluxed in strongly acid solution with a known excess of potas-sium dichromate (K2Cr2O7). After digestion, the remaining un-reduced K2Cr2O7 is titrated with ferrous ammonium sulfate todetermine the amount of K2Cr2O7 consumed and the oxidizablematter is calculated in terms of oxygen equivalent. Keep ratios ofreagent weights, volumes, and strengths constant when samplevolumes other than 50 mL are used. The standard 2-h reflux timemay be reduced if it has been shown that a shorter period yieldsthe same results. Some samples with very low COD or withhighly heterogeneous solids content may need to be analyzed inreplicate to yield the most reliable data. Results are furtherenhanced by reacting a maximum quantity of dichromate, pro-vided that some residual dichromate remains.

2. Apparatus

a. Reflux apparatus, consisting of 500- or 250-mL erlenmeyerflasks with ground-glass 24/40 neck and 300-mm jacket Liebig,West, or equivalent condenser with 24/40 ground-glass joint, anda hot plate having sufficient power to produce at least 1.4 W/cm2of heating surface, or equivalent.

b. Blender.c. Pipets, Class A and wide-bore.

3. Reagents

a. Standard potassium dichromate solution, 0.04167M: Dis-solve 12.259 g K2Cr2O7, primary standard grade, previouslydried at 150C for 2 h, in distilled water and dilute to 1000 mL.This reagent undergoes a six-electron reduction reaction; theequivalent concentration is 6 0.04167M or 0.2500N.

b. Sulfuric acid reagent: Add Ag2SO4, reagent or technicalgrade, crystals or powder, to conc H2SO4 at the rate of 5.5 gAg2SO4/kg H2SO4. Let stand 1 to 2 d to dissolve. Mix.

c. Ferroin indicator solution: Dissolve 1.485 g 1,10-phenan-throline monohydrate and 695 mg FeSO4 7H2O in distilledwater and dilute to 100 mL. This indicator solution may bepurchased already prepared.*

d. Standard ferrous ammonium sulfate (FAS) titrant, approx-imately 0.25M: Dissolve 98 g Fe(NH4)2(SO4)2 6H2O in dis-tilled water. Add 20 mL conc H2SO4, cool, and dilute to 1000mL. Standardize this solution daily against standard K2Cr2O7solution as follows:

Dilute 25.00 mL standard K2Cr2O7 to about 100 mL. Add 30mL conc H2SO4 and cool. Titrate with FAS titrant using 0.10 to0.15 mL (2 to 3 drops) ferroin indicator.

Molarity of FAS solution

Volume 0.04167M K2Cr2O7 solution titrated, mL

Volume FAS used in titration, mL 0.2500

e. Mercuric sulfate, HgSO4, crystals or powder.f. Sulfamic acid: Required only if the interference of nitrites is

to be eliminated (see 5220A.2 above).g. Potassium hydrogen phthalate (KHP) standard,

HOOCC6H4COOK: Lightly crush and then dry KHP to constantweight at 110C. Dissolve 425 mg in distilled water and dilute to1000 mL. KHP has a theoretical COD1 of 1.176 mg O2/mg andthis solution has a theoretical COD of 500 g O2/ mL. Thissolution is stable when refrigerated, but not indefinitely. Be alertto development of visible biological growth. If practical, prepareand transfer solution under sterile conditions. Weekly prepara-tion usually is satisfactory.

4. Procedure

a. Treatment of samples with COD of 50 mg O2/L: Blendsample if necessary and pipet 50.00 mL into a 500-mL refluxingflask. For samples with a COD of 900 mg O2/L, use a smallerportion diluted to 50.00 mL. Add 1 g HgSO4, several glassbeads, and very slowly add 5.0 mL sulfuric acid reagent, withmixing to dissolve HgSO4. Cool while mixing to avoid possibleloss of volatile materials. Add 25.00 mL 0.04167M K2Cr2O7solution and mix. Attach flask to condenser and turn on coolingwater. Add remaining sulfuric acid reagent (70 mL) throughopen end of condenser. Continue swirling and mixing whileadding sulfuric acid reagent. CAUTION: Mix reflux mixture thor-oughly before applying heat to prevent local heating of flaskbottom and a possible blowout of flask contents.

Cover open end of condenser with a small beaker to preventforeign material from entering refluxing mixture and reflux for 2 h.Cool and wash down condenser with distilled water. Disconnectreflux condenser and dilute mixture to about twice its volume with* GFS Chemicals, Inc., Columbus, OH, or equivalent.

CHEMICAL OXYGEN DEMAND (COD) (5220)/Open Reflux Method 5-15

distilled water. Cool to room temperature and titrate excessK2Cr2O7 with FAS, using 0.10 to 0.15 mL (2 to 3 drops) ferroinindicator. Although the quantity of ferroin indicator is not critical,use the same volume for all titrations. Take as the end point of thetitration the first sharp color change from blue-green to reddishbrown that persists for 1 min or longer. Duplicate determinationsshould agree within 5% of their average. Samples with suspendedsolids or components that are slow to oxidize may require additionaldeterminations. The blue-green may reappear. In the same manner,reflux and titrate a blank containing the reagents and a volume ofdistilled water equal to that of sample.

b. Alternate procedure for low-COD samples: Follow procedureof 4a, with two exceptions: (i) use standard 0.004167M K2Cr2O7,and (ii) titrate with standardized 0.025M FAS. Exercise extremecare with this procedure because even a trace of organic matter onthe glassware or from the atmosphere may cause gross errors. If afurther increase in sensitivity is required, concentrate a larger vol-ume of sample before digesting under reflux as follows: Add allreagents to a sample larger than 50 mL and reduce total volume to150 mL by boiling in the refluxing flask open to the atmospherewithout the condenser attached. Compute amount of HgSO4 to beadded (before concentration) on the basis of a weight ratio of 10:1,HgSO4:Cl, using the amount of Cl present in the original volumeof sample. Carry a blank reagent through the same procedure. Thistechnique has the advantage of concentrating the sample withoutsignificant losses of easily digested volatile materials. Hard-to-digest volatile materials such as volatile acids are lost, but animprovement is gained over ordinary evaporative concentrationmethods. Duplicate determinations are not expected to be as preciseas in 5220B.4a.

c. Determination of standard solution: Evaluate the techniqueand quality of reagents by conducting the test on a standardpotassium hydrogen phthalate solution.

5. Calculation

COD as mg O2/L (A B) M 8000

mL sample

where:A mL FAS used for blank,B mL FAS used for sample,M molarity of FAS, and

8000 milliequivalent weight of oxygen 1000 mL/L.

6. Precision and Bias

A set of synthetic samples containing potassium hydrogenphthalate and NaCl was tested by 74 laboratories. At a COD of200 mg O2/L in the absence of chloride, the standard deviationwas 13 mg/L (coefficient of variation, 6.5%). At COD of 160mg O2/L and 100 mg Cl/L, the standard deviation was 14mg/L (coefficient of variation, 10.8%).

7. Reference

1. PITWELL, L.R. 1983. Standard COD. Chem. Brit. 19:907.

8. Bibliography

MOORE, W.A., R.C. KRONER & C.C. RUCHHOFT. 1949. Dichromate refluxmethod for determination of oxygen consumed. Anal. Chem. 21:953.

MEDALIA, A.I. 1951. Test for traces of organic matter in water. Anal.Chem. 23:1318.

MOORE, W.A., F.J. LUDZACK & C.C. RUCHHOFT. 1951. Determination ofoxygen-consumed values of organic wastes. Anal. Chem. 23:1297.

DOBBS, R.A. & R.T. WILLIAMS. 1963. Elimination of chloride interfer-ence in the chemical oxygen demand test. Anal. Chem. 35:1064.

5220 C. Closed Reflux, Titrimetric Method


a. Principle: See 5220B.1a.b. Interferences and limitations: See 5220A.2. Volatile organic

compounds are more completely oxidized in the closed systembecause of longer contact with the oxidant. Before each use inspectculture-tube caps for breaks in the TFE liner. Select culture-tubesize according to block heater capacity and degree of sensitivitydesired. Use the 25- 150-mm tube for samples with low CODcontent because a larger volume sample can be treated.

This procedure is applicable to COD values between 40 and400 mg/L. Obtain higher values by dilution. Alternatively, usehigher concentrations of dichromate digestion solution to deter-mine greater COD values. COD values of 100 mg/L or less canbe obtained by using a more dilute dichromate digestion solutionor a more dilute FAS titrant. Overall accuracy can be improvedby using an FAS titrant which is less than the 0.10M solutionspecified below. Higher dichromate concentrations or reducedFAS concentrations probably require titrations to be done in a

separate vessel, rather than in the digestion vessel, because of thevolumes of titrant required.

2. Apparatus

a. Digestion vessels: Preferably use borosilicate culture tubes,16- 100-mm, 20- 150-mm, or 25- 150-mm, with TFE-lined screw caps. Alternatively, use borosilicate ampules, 10-mLcapacity, 19- to 20-mm diam.

Digestion vessels with premixed reagents and other accesso-ries are available from commercial suppliers. Contact supplierfor specifications.*

b. Block heater or similar device to operate at 150 2C,with holes to accommodate digestion vessels. Use of culturetubes probably requires the caps to be outside the vessel toprotect caps from heat. CAUTION: Do not use an oven because of

* Hach Co., Bioscience, Inc., or equivalent.


the possibility of leaking samples generating a corrosive andpossibly explosive atmosphere. Also, culture tube caps may notwithstand the 150C temperature in an oven.

c. Microburet.d. Ampule sealer: Use only a mechanical sealer to insure

strong, consistent seals.

3. Reagents

a. Standard potassium dichromate digestion solution,0.01667M: Add to about 500 mL distilled water 4.903 gK2Cr2O7, primary standard grade, previously dried at 150C for2 h, 167 mL conc H2SO4, and 33.3 g HgSO4. Dissolve, cool toroom temperature, and dilute to 1000 mL.

b. Sulfuric acid reagent: See Section 5220B.3b.c. Ferroin indicator solution: See Section 5220B.3c. Dilute

this reagent by a factor of 5 (1 4).d. Standard ferrous ammonium sulfate titrant (FAS), approx-

imately 0.10M: Dissolve 39.2 g Fe(NH4)2(SO4)2 6H2O in dis-tilled water. Add 20 mL conc H2SO4, cool, and dilute to 1000mL. Standardize solution daily against standard K2Cr2O7 diges-tion solution as follows:

Pipet 5.00 mL digestion solution into a small beaker. Add 10mL reagent water to substitute for sample. Cool to room tem-perature. Add 1 to 2 drops diluted ferroin indicator and titratewith FAS titrant.

Molarity of FAS solution

Volume 0.01667M K2Cr2O7 solution titrated, mL

Volume FAS used in titration, mL 0.1000

e. Sulfamic acid: See Section 5220B.3f.f. Potassium hydrogen phthalate standard: See Section

5220B.3g.

4. Procedure

Wash culture tubes and caps with 20% H2SO4 before first useto prevent contamination. Refer to Table 5220:I for propersample and reagent volumes. Make volumetric measurements asaccurate as practical; use Class A volumetric ware. The mostcritical volumes are of the sample and digestion solution. Use amicroburet for titrations. Measure H2SO4 to 0.1 mL. The useof hand-held pipettors with non-wetting (polyethylene) pipet tipsis practical and adequate. Place sample in culture tube or ampuleand add digestion solution. Carefully run sulfuric acid reagentdown inside of vessel so an acid layer is formed under thesample-digestion solution layer. Tightly cap tubes or seal am-pules, and invert each several times to mix completely. CAUTION:Wear face shield and protect hands from heat produced whencontents of vessels are mixed. Mix thoroughly before applyingheat to prevent local heating of vessel bottom and possibleexplosive reaction.

Place tubes or ampules in block digester preheated to 150Cand reflux for 2 h behind a protective shield. CAUTION: Thesesealed vessels may be under pressure from gases generatedduring digestion. Wear face and hand protection when handling.If sulfuric acid is omitted or reduced in concentration, very high

and dangerous pressures will be generated at 150C. Cool toroom temperature and place vessels in test tube rack. Somemercuric sulfate may precipitate out but this will not affect theanalysis. Remove culture tube caps and add small TFE-coveredmagnetic stirring bar. If ampules are used, transfer contents to alarger container for titrating. Add 0.05 to 0.10 mL (1 to 2 drops)ferroin indicator and stir rapidly on magnetic stirrer while titrat-ing with standardized 0.10M FAS. The end point is a sharp colorchange from blue-green to reddish brown, although the blue-green may reappear within minutes. In the same manner refluxand titrate a blank containing the reagents and a volume ofdistilled water equal to that of the sample.

5. Calculation

COD as mg O2/L (AB) M 8000

mL sample

where:A mL FAS used for blank,B mL FAS used for sample,M molarity of FAS, and

8000 milliequivalent weight of oxygen 1000 mL/L.

Preferably analyze samples in duplicate because of smallsample size. Samples that are inhomogeneous may require mul-tiple determinations for accurate analysis. Results should agreewithin 5% of their average unless the condition of the sampledictates otherwise.


Sixty synthetic samples containing potassium hydrogen phthalateand NaCl were tested by six laboratories. At an average COD of195 mg O2/L in the absence of chloride, the standard deviation was11 mg O2/L (coefficient of variation, 5.6%). At an average CODof 208 mg O2/L and 100 mg Cl/L, the standard deviation was10mg O2/L (coefficient of variation, 4.8%).

TABLE 5220:I. SAMPLE AND REAGENT QUANTITIES FOR VARIOUSDIGESTION VESSELS

Digestion VesselSample

mL

DigestionSolution

mL

SulfuricAcid

ReagentmL

TotalFinal

VolumemL

Culture tubes:16 100 mm 2.50 1.50 3.5 7.520 150 mm 5.00 3.00 7.0 15.025 150 mm 10.00 6.00 14.0 30.0

Standard 10-mLampules 2.50 1.50 3.5 7.5

CHEMICAL OXYGEN DEMAND (COD) (5220)/Closed Reflux, Titrimetric Method 5-17

5220 D. Closed Reflux, Colorimetric Method


a. Principle: See Section 5220B.1a. When a sample is di-gested, the dichromate ion oxidizes COD material in the sample.This results in the change of chromium from the hexavalent (VI)state to the trivalent (III) state. Both of these chromium speciesare colored and absorb in the visible region of the spectrum. Thedichromate ion (Cr2O72) absorbs strongly in the 400-nm region,where the chromic ion (Cr3) absorption is much less. Thechromic ion absorbs strongly in the 600-nm region, where thedichromate has nearly zero absorption. In 9M sulfuric acidsolution, the approximate molar extinction coefficients for thesechromium species are as follows: Cr3 50 L/mole cm at 604nm; Cr2O72 380 L/mole cm at 444 nm; Cr3 25 L/molecm at 426 nm. The Cr3 ion has a minimum in the region of 400nm. Thus a working absorption maximum is at 420 nm.

For COD values between 100 and 900 mg/L, increase in Cr3 inthe 600-nm region is determined. Higher values can be obtained bysample dilution. COD values of 90 mg/L or less can be determinedby following the decrease in Cr2O72 at 420 nm. The correspondinggeneration of Cr3 gives a small absorption increase at 420 nm, butthis is compensated for in the calibration procedure.

b. Interferences and limitations: See Section 5220C.1b.For this procedure to be applicable, all visible light-absorbing

interferents must be absent or be compensated for. This includesinsoluble suspended matter as well as colored components. Ifeither type of interference occurs, the test is not necessarily lostbecause COD can be determined titrimetrically as in 5220C.

2. Apparatus

a. See Section 5220C.2. Ensure that reaction vessels are ofoptical quality. Other types of absorption cells with varying pathlengths may be used. Use the extinction coefficients of the ionsof interest for this approach.

b. Spectrophotometer, for use at 600 nm and/or 420 nm withaccess opening adapter for ampule or 16-, 20-, or 25-mm tubes.Verify that the instrument operates in the region of 420 nm and600 nm. Values slightly different from these may be found,depending on the spectral bandpass of the instrument.

3. Reagents

a. Digestion solution, high range: Add to about 500 mL distilledwater 10.216 g K2Cr2O7, primary standard grade, previously driedat 150C for 2 h, 167 mL conc H2SO4, and 33.3 g HgSO4. Dissolve,cool to room temperature, and dilute to 1000 mL.

b. Digestion solution, low range: Prepare as in 3a, but useonly 1.022 g potassium dichromate.

c. Sulfuric acid reagent: See Section 5220B.3b.d. Sulfamic acid: See Section 5220B.3f.e. Potassium hydrogen phthalate standard: See Section

5220B.3g.

4. Procedure

a. Treatment of samples: Measure suitable volume of sampleand reagents into tube or ampule as indicated in Table 5220:I.

Prepare, digest, and cool samples, blank, and one or morestandards as directed in Section 5220C.4. Note the safety pre-cautions. It is critical that the volume of each component beknown and that the total volume be the same for each reactionvessel. If volumetric control is difficult, transfer digested sample,dilute to a known volume, and read. Premixed reagents indigestion tubes are available commercially.

b. Measurement of dichromate reduction: Cool sample toroom temperature slowly to avoid precipitate formation. Oncesamples are cooled, vent, if necessary, to relieve any pressuregenerated during digestion. Mix contents of reaction vessels tocombine condensed water and dislodge insoluble matter. Letsuspended matter settle and ensure that optical path is clear.Measure absorption of each sample blank and standard at se-lected wavelength (420 nm or 600 nm). At 600 nm, use anundigested blank as reference solution. Analyze a digested blankto confirm good analytical reagents and to determine the blankCOD; subtract blank COD from sample COD. Alternately, usedigested blank as the reference solution once it is established thatthe blank has a low COD.

At 420 nm, use reagent water as a reference solution. Measureall samples, blanks, and standards against this solution. Theabsorption measurement of an undigested blank containing di-chromate, with reagent water replacing sample, will give initialdichromate absorption. Any digested sample, blank, or standardthat has a COD value will give lower absorbance because of thedecrease in dichromate ion. Analyze a digested blank with re-agent water replacing sample to ensure reagent quality and todetermine the reagents contribution to the decrease in absor-bance during a given digestion. The difference between absor-bances of a given digested sample and the digested blank is ameasure of the sample COD. When standards are run, plotdifferences of digested blank absorbance and digested standardabsorbance versus COD values for each standard.

c. Preparation of calibration curve: Prepare at least five stan-dards from potassium hydrogen phthalate solution with CODequivalents to cover each concentration range. Make up to vol-ume with reagent water; use same reagent volumes, tube, orampule size, and digestion procedure as for samples. Preparecalibration curve for each new lot of tubes or ampules or whenstandards prepared in 4a differ by5% from calibration curve.Curves should be linear. However, some nonlinearity may occur,depending on instrument used and overall accuracy needed.

5. Calculation

If samples, standards, and blanks are run under same conditionsof volume and optical path length, calculate COD as follows:

COD as mg O2/L mg O2 in final volume 1000

mL sample

Preferably analyze samples in duplicate because of smallsample size. Samples that are inhomogeneous may require mul-tiple determinations for accurate analysis. These should notdiffer from their average by more than 5% for the high-levelCOD test unless the condition of the sample dictates otherwise.


In the low-level procedure, results below 25 mg/L may tend to bequalitative rather than quantitative.


Forty-eight synthetic samples containing potassium hydrogenphthalate and NaCl were tested by five laboratories. At anaverage COD of 193 mg O2/L in the absence of chloride, thestandard deviation was 17 mg O2/L (coefficient of variation8.7%). At an average COD of 212 mg O2/L and 100 mg Cl/L,the standard deviation was 20 mg O2/L (coefficient of varia-tion, 9.6%). Additional QA/QC data for both high- and low-levelprocedures may be found elsewhere.1

7. Reference

1. AMERICAN SOCIETY FOR TESTING AND MATERIALS. 1995. Standard testmethods for chemical oxygen demand (dichromate oxygen demand)of water. D1252-95, ASTM Annual Book of Standards. AmericanSoc. Testing & Materials, Philadelphia, Pa.

8. Bibliography

JIRKA, A.M. & M.J. CARTER. 1975. Micro semi-automated analysis ofsurface and wastewaters for chemical oxygen demand. Anal. Chem.47:1397.

HIMEBAUGH, R.R. & M.J. SMITH. 1979. Semi-micro tube method forchemical oxygen demand. Anal. Chem. 51:1085.

TOTAL ORGANIC CARBON (TOC) (5310)/Introduction 5-19

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SM+5220+COD_20th+ed